This invention relates to an improved process for preparing oxo alcohols, and more specifically to an improved process for preparing such alcohols at a higher yield by reducing the concentration of impurities which represent an irreversible yield loss.
The oxo process is the commercial application of the hydroformylation reaction for making higher alcohols from olefins. In the oxo process, an olefin reacts with carbon monoxide and hydrogen at elevated temperature and pressure in the presence of a transition metal catalyst, typically a cobalt or rhodium carbonyl complex, to produce a hydroformylation reaction product intermediate. This intermediate is predominantly two isomeric aldehydes as illustrated by the following chemical reaction: ##STR1## Following removal of the transition metal catalyst from the hydroformylation reaction product, the demetalled intermediate is converted by hydrogenation to alcohols.
The demetalled hydroformylation reaction product intermediate contains not only higher aldehydes, but also higher alcohols, unreacted olefin and secondary products. The secondary products include acetal and formate impurities. The acetal impurities will undergo chemical reaction during hydrogenation of the demetalled reaction product intermediate as illustrated by the following reaction: ##STR2##
As illustrated by this reaction, the acetal impurities are converted to higher alcohol and undesired dimers. The dimers are not readily converted to the desired higher alcohol and thus represent an irreversible yield loss.
The formate impurities also undergo chemical reaction during hydrogenation of the demetalled intermediate as illustrated by the following reaction: ##STR3##
The formate impurities are converted to higher alcohol and methanol. Methanol is an undesired product because it is difficult to remove from product streams and it is difficult to treat in conventional anaerobic waste treatment plants.
Efforts have been made to reduce the adverse effects of acetal and formate impurities in the hydroformylation reaction product. U.S. Pat. Nos. 4,658,068; 4,656,215 and 4,683,343 each describe distilling the demetalled hydroformylation reaction product after hydrogenation to separate the desired alcohol from a higher boiling heavy oxo fraction, which would contain the acetal and formate impurities. This heavy oxo fraction is subsequently subjected to steam cracking in the presence of an active metal oxide or pseudo-metal oxide catalyst at elevated temperatures to form a cracked mixture containing an increased concentration of higher alcohol and aldehyde. The cracked mixture can then be recycled to the hydroformylation or hydrogenation steps of the process. Although this process significantly reduces the adverse consequences of the acetal and formate impurities in the hydrogenated reaction product, it would be desirable to utilize a process that reduces the concentration of these impurities before the hydrogenation step.
To that end, U.S. Pat. Nos. 4,401,834 (King patent) and 3,935,285 (Tummes patent) each disclose hydrolyzing the demetalled hydroformylation reaction product prior to hydrogenation in an attempt to convert a significant amount of the acetal and formate impurities to higher alcohol and aldehyde, as well as formic acid. Formic acid can be decomposed during hydrogenation to hydrogen and carbon dioxide. The hydrolysis reaction for the acetal and formate impurities and the subsequent decomposition of formic acid can be illustrated as follows: ##STR4##
The King patent discloses hydrolyzing the demetalled hydroformylation reaction product in the absence of added acidic compounds at elevated temperatures and pressures. Although some conversion of the impurities to higher aldehydes and alcohols is achieved, the slight increase in conversion does not offset the time and expense required to carry out an additional hydrolysis step in the oxo process.
The Tummes patent discloses hydrolyzing the demetalled product in the presence of a high surface area alumina catalyst at elevated temperatures and essentially atmospheric pressure. Although slightly higher conversions may be achieved using the high surface area alumina catalyst relative to an uncatalyzed hydrolysis, the conversion is still inadequate. Additionally, the Tummes hydrolysis requires extremely high temperatures, which is uneconomical because of dramatically increased energy requirements, and a concentration of water or steam greater than the solubility limit of the water in the hydrocarbon phase. Therefore, an additional phase separation would be required to separate the water from the desired product.
In view of the deficiencies of the prior art, an improved oxo process is needed which would reduce the concentration of acetal and formate impurities in the demetalled hydroformylation reaction product prior to hydrogenation. More specifically, an improved process for increasing the concentration of oxo alcohols is needed which converts a substantial amount of acetal and formate impurities in the demetalled product to higher aldehyde and alcohol prior to hydrogenation.